Port scenegraph autotest case for alpha batching problem

This should work as-is in 6.1 and newer, so it needs no specific
fix. The autotest case added in 5.15(.5) is useful however to
prevent future regressions, so port that from the 5.15 LTS commit
a6705dc99a81b7eab35db61957963d375e723fce.

Task-number: QTBUG-92984
Change-Id: Ifd6f807cc024395e8fe60ecf19785dd98152bbb5
Reviewed-by: Qt CI Bot <qt_ci_bot@qt-project.org>
Reviewed-by: Eskil Abrahamsen Blomfeldt <eskil.abrahamsen-blomfeldt@qt.io>

4 files changed, 117 insertions, 1 deletions

diff --git a/tests/auto/quick/scenegraph/data/render_AlphaOverlapRebuild.qml b/tests/auto/quick/scenegraph/data/render_AlphaOverlapRebuild.qml
new file mode 100644
index 0000000000..0d5fae3f70
--- /dev/null
+++ b/tests/auto/quick/scenegraph/data/render_AlphaOverlapRebuild.qml
@@ -0,0 +1,115 @@
+/****************************************************************************
+**
+** Copyright (C) 2021 The Qt Company Ltd.
+** Contact: https://www.qt.io/licensing/
+**
+** This file is part of the test suite of the Qt Toolkit.
+**
+** $QT_BEGIN_LICENSE:GPL-EXCEPT$
+** Commercial License Usage
+** Licensees holding valid commercial Qt licenses may use this file in
+** accordance with the commercial license agreement provided with the
+** Software or, alternatively, in accordance with the terms contained in
+** a written agreement between you and The Qt Company. For licensing terms
+** and conditions see https://www.qt.io/terms-conditions. For further
+** information use the contact form at https://www.qt.io/contact-us.
+**
+** GNU General Public License Usage
+** Alternatively, this file may be used under the terms of the GNU
+** General Public License version 3 as published by the Free Software
+** Foundation with exceptions as appearing in the file LICENSE.GPL3-EXCEPT
+** included in the packaging of this file. Please review the following
+** information to ensure the GNU General Public License requirements will
+** be met: https://www.gnu.org/licenses/gpl-3.0.html.
+**
+** $QT_END_LICENSE$
+**
+****************************************************************************/
+
+import QtQuick
+
+/*
+    QTBUG-92984.
+
+    Have three Image (with semi-transparency) and two semi-transparent
+    Rectangle elements, and so all in the alpha render list, with images that
+    are big enough to not get atlased. (meaning the underlying nodes never get
+    merged, but the nodes for the Rectangle elements might)
+
+    Lay them out vertically below each other, with the two Rectangles on top of
+    the second and third Images, respectively. Then change (swap) the source
+    property of the Images. This triggers a rebuild in the batch renderer.
+
+    Verify that the results are still correct, i.e. that the two Rectangle
+    elements do not get merged. An incorrect result would be having the third
+    Image rendered on top of the corresponding Rectangle due the two Rectangles
+    (incorrectly) being in one merged batch. The Image should always be below,
+    regardless of which nodes get changed, invalidated, and how batches get
+    rebuilt.
+
+    The base-final sample set 1 just verifies that the Image changes from the
+    bluish to greenish. The important part is the second set of samples: this
+    checks that the red(ish) rectangle is still on top of the third Image. With
+    incorrect merging behavior the second final result would be the same as the
+    first final one (i.e. the "background" Image rendered, incorrectly, on top
+    of the Rectangle).
+
+    #samples: 4
+                 PixelPos     R         G          B           Error-tolerance
+    #base:        30  115    0.24313   0.30588    0.99607       0.05
+    #base:        30  124    0.847059  0.062745   0.2           0.05
+    #final:       30  115    0.36078   0.99607    0.42745       0.05
+    #final:       30  124    0.870588  0.2        0.0862745     0.05
+*/
+
+RenderTestBase {
+    id: root
+
+    property string selectedItem: "item2"
+
+    Item {
+        width: 150; height: 50
+        Image {
+            width: parent.width
+            objectName: "item1"
+            source: "widebtn1.png"
+        }
+    }
+
+    Item {
+        y: 50; width: 150; height: 50
+        Image {
+            width: parent.width
+            objectName: "item2"
+            source: selectedItem == objectName ? "widebtn2.png" : "widebtn1.png"
+        }
+        Rectangle {
+            anchors.fill: parent
+            anchors.margins: 20
+            color: "red"
+            opacity: 0.8
+        }
+    }
+
+    Item {
+        y: 100; width: 150; height: 50
+        Image {
+            id: img3
+            width: parent.width
+            objectName: "item3"
+            source: selectedItem == objectName ? "widebtn2.png" : "widebtn1.png"
+        }
+        Rectangle {
+            width: parent.width + 50
+            anchors.centerIn: parent
+            height: img3.height - 40
+            color: "red"
+            opacity: 0.8
+        }
+    }
+
+    onEnterFinalStage: {
+        selectedItem = "item3";
+        finalStageComplete = true;
+    }
+}
diff --git a/tests/auto/quick/scenegraph/data/widebtn1.png b/tests/auto/quick/scenegraph/data/widebtn1.png
new file mode 100644
index 0000000000..1150b67a7a
--- /dev/null
+++ b/tests/auto/quick/scenegraph/data/widebtn1.png
diff --git a/tests/auto/quick/scenegraph/data/widebtn2.png b/tests/auto/quick/scenegraph/data/widebtn2.png
new file mode 100644
index 0000000000..40afe08363
--- /dev/null
+++ b/tests/auto/quick/scenegraph/data/widebtn2.png
diff --git a/tests/auto/quick/scenegraph/tst_scenegraph.cpp b/tests/auto/quick/scenegraph/tst_scenegraph.cpp
index 4f34ed9fae..9803412703 100644
--- a/tests/auto/quick/scenegraph/tst_scenegraph.cpp
+++ b/tests/auto/quick/scenegraph/tst_scenegraph.cpp
@@ -376,7 +376,8 @@ void tst_SceneGraph::render_data()
           << "render_ImageFiltering.qml"
           << "render_bug37422.qml"
           << "render_OpacityThroughBatchRoot.qml"
-          << "render_Mipmap.qml";
+          << "render_Mipmap.qml"
+          << "render_AlphaOverlapRebuild.qml";
 
     QRegularExpression sampleCount("#samples: *(\\d+)");
     //                          X:int   Y:int   R:float       G:float       B:float       Error:float